Noob - world color affecting rendered (plane) image
The reason for this is that the world color does not only define the background, but also acts as a light source. When you set it to orange, you have a purple object illuminated with orange light, causing the red end result.
Using the white world color will illuminate it with white light, allowing the object to stay purple.

You could have a look here for a method to use a white lighting while having a colored background.

I've uploaded an example. The only changes I made that aren't directly visible when you open the file is that I enabled "Transparent" in the Film tab under the Render panel.